This invention relates to the formation of solder balls used in semiconductor devices such as a Ball Grid Array device, and more particularly to the structure and method of forming defect-free solder ball contacts.
Ball Grid Array (BGA) semiconductor devices utilize a solder ball as the contact between the device and a mounting surface. In forming solder ball contacts, the solder ball is usually formed on a solid surface using a solder preform, and the solder ball is formed as the solder form is reflowed to form the spherical solder ball contact. When the BGA device is mounted, usually on a printed circuit board, the device is placed on an array of contact pads of solder, or a contact pad with a coating of solder thereon, with one solder ball contact on each pad. The circuit board, with the BGA device mounted thereon, is subjected to a solder reflow process in which a portion of the contact pad, or solder coating thereon, and solder ball contact partially melt, bonding the device to the circuit board.
Printed circuit boards have vias which interconnect various conductors on different layers of the circuit board. These vias usually extend completely through the circuit board layers forming an opening through the circuit board. Normally the board and contacts thereon are laid out so that a solder ball contact of the BGA device does not coincide with a via. However, as circuit boards become smaller with a higher density of components, at times it is necessary to place the device solder ball contacts over vias. Also, with devices with smaller pitch solder balls, the size of the solder ball becomes smaller while the via hole may remain the same size. The placement of a solder ball contact over a via often causes the solder ball contact to collapse during solder reflow since a portion of the solder ball contact flows into the via.
A similar problem is addressed in U.S. Pat. No. 5,511,305, wherein, in wave soldering processes, heat induced into the via and the solder joints of the solder ball contacts of BGA devices causes defects in the solder joints between the solder ball contact and the contact pad to which the solder ball is soldered. This problem is addressed by covering the via on the underside of the printed circuit board with an insulating material to shield the via from excessive heat during the wave soldering process. This reduces the heat flow through the via to the BGA device contacts on the surface of the printed circuit board.
The invention is a method for attaching an electronic component having Ball Grid Array contacts to a circuit board contact array to prevent the solder balls of the ball grid array from fracturing and distorting during solder reflow when the ball grid array contact is attached to a contact on a printed circuit board that has a via extending at least partially though the printed circuit board. A solder form is placed over each via in each contact of the contact array. The electronic component that has BGA contacts is placed over the contact array such that each ball of the ball grid array of the electronic component resides on a solder form. The component and circuit is subjected to a solder reflow process to seal the component to the circuit board. The solder form at least partially fills the via preventing the BGA contact from collapsing into the via.